Musnad Ahmad 658:111

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from al-Rukayn ibn al-Rabi', he said: I heard 'Adi ibn Thabit narrating from al-Bara' ibn 'Azib, he said: Some people passed by us while they were departing, so we said: "Where are you going?" They said: "The Messenger of Allah, peace and blessings of Allah be upon him, sent us to a man who went to his father's wife, so that we may kill him."